This invention relates to a plastification and injection device for an injection molding machine, the device having a drive which, during plastification, causes the extruder screw to rotate about its central axis, and having a pressurized injection piston and cylinder unit controlled by a directional control valve for axially shifting the screw for injecting plasticized material into the machine.
The plastic or plasticized material as herein referred includes thermoplasts, duroplasts, and elastomers/rubber.
The general type of plastification and injection device to which the invention relates, is known in construction and operation. The device usually includes a screw housing having an outlet nozzle and an inlet for plastic material granules, and the plastification and injection screw is mounted within the housing with its tip end spaced from the outlet to define a chamber for the plasticized material. A hydromotor is usually provided for rotating the screw about its central axis for plasticizing the granular material fed into the housing, and a pressurized piston and cylinder unit is provided for axially shifting the screw for injecting the plasticized material into the space between the mating molding parts of the molding machine. During this operation, at the end of the piston suction stroke within the injection cylinder, the pressurized working fluid discharged from the injection cylinder is subjected to a so-called back pressure and, to fabricate the molded article, the injection cylinder is pressurized such that the screw, now operating as a plunger, moves forwardly and at the same time injects plasticized material into the injection mold. After the mold is filled, a holding pressure is applied to take into account the cooling of the molded article which takes place under thermodynamic conditions.
Compensation must be made for the shrinkage of molded articles on cooling. This was heretofore carried out by maintaining a sufficiently high holding pressure, so that through a slight axial displacement of the screw material could be injected into the mold. However, such control mechanisms are usually cumbersome and uneconomical, and it has been found that shrinkage of the mold article can thereby be compensated only incompletely since the operating temperature of the injection molding machine and, thereby, the viscosity of the plasticized material to be injected, fluctuates.
Another known technique in producing accurately sized and error-free mold articles is to minimize the measurement and weight variations of the molded articles so that, during the changeover from injection pressure to holding pressure, discontinuities in the actual pressure variation versus a pre-programmed pressure variation do not occur. Such is disclosed in West German Pat. No. 25 33 303.
As will be seen, the present invention adopts the principle of a hydraulically rigid feedback loop in accordance with the principle of an electro-hydraulic linear amplifier. Hydraulic components operating in accordance with this principle are known from the publication of Hartmann and Lammle KG, "Electrohydraulic Linear Amplifier LVS," 10th edition, 1977. However, these components have heretofore only been employed for the exact positioning of machine components that can slide on machine tools, for example.
And, plastification and injection devices are disclosed in DE-AS No. 25 41 733 and DE-OS No. 21 55 130 in which an axially non-returning is provided within the screw housing between the tip of the screw and the annular space surrounding the screw. The ring forms a back flow stop with the screw together with a retaining ring. The axial movement of the non-return ring is delimited on one side by an adjacent conical shoulder of the screw tip having crown grooves, and on the other by the leading edge of the screw thread. During plastification, the ring bears on the conical shoulder of the screw tip, and the plasticized material flows into the plastification chamber downstream of the screw tip through an annular gap defined between the ring and the conical shoulder as well as through the grooves of the conical shoulder beyond the screw tip. During injection, the non-return ring bears sealingly against the end face of the screw thread and prevents plastified material from flowing back along the lands or ridges of the screw.
When fabricating injection-molded precision articles, extremely small quantities of weight variations are induced, partly because of the fact that when the injection pressure is being applied, the small amounts of plastified material flow back from the area of the screw tip through the open annular gap between the lands or ridges of the screw and the inner wall of the screw housing. Since the viscosity of the plastified material depends, among other things, the temperature, it can be seen that continually fluctuating amounts of material, however small, flow back and deliberately affect the precision parts.